FreeBSD Man Pages

SA(4) FreeBSD Kernel Interfaces Manual SA(4)
NAMEsa -- SCSI Sequential Access device driver
SYNOPSISdevicesaDESCRIPTION
The sa driver provides support for all SCSI devices of the sequential
access class that are attached to the system through a supported SCSI
Host Adapter. The sequential access class includes tape and other linear
access devices.
A SCSI Host adapter must also be separately configured into the system
before a SCSI sequential access device can be configured.
MOUNT SESSIONS
The sa driver is based around the concept of a ``mountsession'', which
is defined as the period between the time that a tape is mounted, and the
time when it is unmounted. Any parameters set during a mount session
remain in effect for the remainder of the session or until replaced. The
tape can be unmounted, bringing the session to a close in several ways.
These include:
1. Closing a `rewind device', referred to as sub-mode 00 below. An
example is /dev/sa0.
2. Using the MTOFFL ioctl(2) command, reachable through the `offline'
command of mt(1).
It should be noted that tape devices are exclusive open devices, except
in the case where a control mode device is opened. In the latter case,
exclusive access is only sought when needed (e.g., to set parameters).
SUB-MODES
Bits 0 and 1 of the minor number are interpreted as `sub-modes'. The
sub-modes differ in the action taken when the device is closed:
00 A close will rewind the device; if the tape has been written, then
a file mark will be written before the rewind is requested. The
device is unmounted.
01 A close will leave the tape mounted. If the tape was written to, a
file mark will be written. No other head positioning takes place.
Any further reads or writes will occur directly after the last
read, or the written file mark.
10 A close will rewind the device. If the tape has been written, then
a file mark will be written before the rewind is requested. On
completion of the rewind an unload command will be issued. The
device is unmounted.
BLOCKING MODES
SCSI tapes may run in either `variable' or `fixed' block-size modes.
Most QIC-type devices run in fixed block-size mode, where most nine-track
tapes and many new cartridge formats allow variable block-size. The dif-
ference between the two is as follows:
Variable block-size: Each write made to the device results in a single
logical record written to the tape. One can never read or write part of
a record from tape (though you may request a larger block and read a
smaller record); nor can one read multiple blocks. Data from a single
write is therefore read by a single read. The block size used may be any
value supported by the device, the SCSI adapter and the system (usually
between 1 byte and 64 Kbytes, sometimes more).
When reading a variable record/block from the tape, the head is logically
considered to be immediately after the last item read, and before the
next item after that. If the next item is a file mark, but it was never
read, then the next process to read will immediately hit the file mark
and receive an end-of-file notification.
Fixed block-size: Data written by the user is passed to the tape as a
succession of fixed size blocks. It may be contiguous in memory, but it
is considered to be a series of independent blocks. One may never write
an amount of data that is not an exact multiple of the blocksize. One
may read and write the same data as a different set of records. In other
words, blocks that were written together may be read separately, and
vice-versa.
If one requests more blocks than remain in the file, the drive will
encounter the file mark. As there is some data to return (unless there
were no records before the file mark), the read will succeed, returning
that data. The next read will return immediately with a value of 0. (As
above, if the file mark is never read, it remains for the next process to
read if in no-rewind mode.)
BLOCK SIZES
By default, the driver will NOT accept reads or writes to a tape device
that are larger than may be written to or read from the mounted tape
using a single write or read request. Because of this, the application
author may have confidence that his wishes are respected in terms of the
block size written to tape. For example, if the user tries to write a
256KB block to the tape, but the controller can handle no more than
128KB, the write will fail. The previous FreeBSD behavior, prior to
FreeBSD 10.0, was to break up large reads or writes into smaller blocks
when going to the tape. The problem with that behavior, though, is that
it hides the actual on-tape block size from the application writer, at
least in variable block mode.
If the user would like his large reads and writes broken up into separate
pieces, he may set the following loader tunables. Note that these tun-
ables WILL GO AWAY in FreeBSD 11.0. They are provided for transition
purposes only.
kern.cam.sa.allow_io_split
This variable, when set to 1, will configure all sa devices to split
large buffers into smaller pieces when needed.
kern.cam.sa.%d.allow_io_split
This variable, when set to 1, will configure the given sa unit to
split large buffers into multiple pieces. This will override the
global setting, if it exists.
There are several sysctl(8) variables available to view block handling
parameters:
kern.cam.sa.%d.allow_io_split
This variable allows the user to see, but not modify, the current I/O
split setting. The user is not permitted to modify this setting so
that there is no chance of behavior changing for the application
while a tape is mounted.
kern.cam.sa.%d.maxio
This variable shows the maximum I/O size in bytes that is allowed by
the combination of kernel tuning parameters (MAXPHYS, DFLTPHYS) and
the capabilities of the controller that is attached to the tape
drive. Applications may look at this value for a guide on how large
an I/O may be permitted, but should keep in mind that the actual max-
imum may be restricted further by the tape drive via the SCSI READ
BLOCK LIMITS command.
kern.cam.sa.%d.cpi_maxio
This variable shows the maximum I/O size supported by the controller,
in bytes, that is reported via the CAM Path Inquiry CCB
(XPT_PATH_INQ). If this is 0, that means that the controller has not
reported a maximum I/O size.
FILE MARK HANDLING
The handling of file marks on write is automatic. If the user has writ-
ten to the tape, and has not done a read since the last write, then a
file mark will be written to the tape when the device is closed. If a
rewind is requested after a write, then the driver assumes that the last
file on the tape has been written, and ensures that there are two file
marks written to the tape. The exception to this is that there seems to
be a standard (which we follow, but do not understand why) that certain
types of tape do not actually write two file marks to tape, but when
read, report a `phantom' file mark when the last file is read. These
devices include the QIC family of devices. (It might be that this set of
devices is the same set as that of fixed block devices. This has not
been determined yet, and they are treated as separate behaviors by the
driver at this time.)
IOCTLS
The sa driver supports all of the ioctls of mtio(4).
FILES
/dev/[n][e]sa[0-9] general form:
/dev/sa0 Rewind on close
/dev/nsa0 No rewind on close
/dev/esa0 Eject on close (if capable)
/dev/sa0.ctl Control mode device (to examine state while another
program is accessing the device, e.g.).
DIAGNOSTICS
None.
SEE ALSOcam(4), mt(1)AUTHORS
The sa driver was written for the CAM SCSI subsystem by Justin T. Gibbs
and Kenneth Merry. Many ideas were gleaned from the st device driver
written and ported from Mach 2.5 by Julian Elischer.
The current owner of record is Matthew Jacob who has suffered too many
years of breaking tape drivers.
BUGS
This driver lacks many of the hacks required to deal with older devices.
Many older SCSI-1 devices may not work properly with this driver yet.
Additionally, certain tapes (QIC tapes mostly) that were written under
FreeBSD 2.X are not automatically read correctly with this driver: you
may need to explicitly set variable block mode or set to the blocksize
that works best for your device in order to read tapes written under
FreeBSD 2.X.
Fine grained density and compression mode support that is bound to spe-
cific device names needs to be added.
Support for fast indexing by use of partitions is missing.
FreeBSD 10.1 August 23, 2013 FreeBSD 10.1